Charles, hands-on grandfather, seen with Prince Louis in new Royal photo

Prince Charles has been photographed with his grandson Louis and the Duchess of Cambridge in a newly-released image.

The photo was released by Buckingham Palace to coincide with this evening's BBC One documentary, Prince, Son and Heir.

In the programme, the Duke of Cambridge spoke of his hopes that the Prince of Wales will spend more time with his grandchildren as he reaches the age of 70, as he and his brother celebrate the landmark birthday with a moving public tribute.

The Duke, whose children Prince George, Princess Charlotte and Prince Louis are the Prince’s only grandchildren until the arrival of the Sussex baby in the spring, said his father was “brilliant” with all three, expressing his wish that they see more of him.

Saying he was "working more heavily" on ensuring the Prince can spend time with his family despite his heavy workload, he added now his father was growing older "we need him there as much as possible".

In a touching tribute to the Prince, the Dukes of Cambridge and Sussex have spoken of their admiration for his work and personal drive, sharing the essential ingredients they believe have shaped the man he is today.

Urging him to "slow down" as he reaches 70, after a lifetime falling asleep at his desk, his sons disclosed how they have absorbed his “OCD” habit of turning lights off to save energy, and their belief that his time eating jam sandwiches with the military helped save him from a “sheltered” life.

Harry and William have urged Prince Charles to slow down as he approaches 70

“It’s something I’m working more heavily on, put it that way,” says the Duke. “I think he does have time for it, but I would like him to have more time with the children.

“I think now he’s reached his 70th year it’s a perfect time to consolidate a little bit because, as most families would do, you are worried about having them around and making sure their health is okay.

Playtime in the garden at BirkhallCredit: Buckingham Palace

“He’s the fittest man I know, but equally I want him to be fit until he’s 95, going on.

“So, having more time with him at home would be lovely, and being able to you know play around with the grandchildren.

“When he’s there, he’s brilliant, but we need him there as much as possible.”

The Prince has previously been rumoured to have been disappointed by his sometimes limited contact with the Cambridge children, who are said to be very close to their mother’s family, the Middletons.

A documentary into his life will be broadcast on BBC One tonight

He has regularly spoken fondly of them in public and is known to have planted a wood at Birkhall, his home in Scotland, as a gift for Prince George’s birth.

The programme is the first time Prince William and Prince Harry have offered such fulsome public praise of their father, with moving words about their admiration for his work and filial concern over his dedication to duty.

“He does need to slow down,” the Duke of Sussex says. “This is a man who has dinner ridiculously late at night, and then goes to his desk later that night and will fall asleep on his notes to the point where he’ll wake up with a piece of paper stuck to his face.

“The man never stops. When we were kids, there were bags and bags and bags of work that the office just sent to him. We could barely even get to his desk to say goodnight to him.”

The Duke of Cambridge adds: “He has amazing personal discipline. He has - and its frustrated me in the past a lot - a routine. The only way to fit all this stuff in is things have to be compartmentalised.”

The Dukes also give an unexpected insight into how their father’s devotion to the environment has rubbed off on them, with Prince Harry disclosing how his wife Meghan has been left baffled by his insistence on turning off lights.

“He’s a stickler for turning lights off, and that’s now something that I’m obsessed with as well,” he says.

Prince Charles, the Prince of Wales, and Camilla, Duchess of Cornwall, in "Prince, Son and Heir: Charles at 70", a documentary which will be broadcast at 2100 on 8th November, 2018, on BBC One.

In a filmed conversation between the brothers, Prince William adds: “I know I’ve got serious OCD on light switches now which is terrible.”

“I go, ‘We only need one light, we don’t need, like, six’ and all of a sudden it becomes a habit.

“Those small habit changes he’s making, every single person can do. And I think is one of the key lessons certainly that I felt that he taught us.”

Asked for their words of advice to the Prince at 70 though, his sons focus tellingly on work-life balance, and their concern that all his hard work over the decades will pay off.

“Please have dinner earlier is my message to him,” the Duke of Sussex says. “I would encourage him to remain optimistic because I think it can be very easy to become despondent and negative.

“But with his children and his grandchildren and hopefully more grandchildren to come, he can get energy the family side and then carry on his leadership role.”

The Duke of Cambridge, laughing, adds: “I’m very pleased that he’s made 70.

“If you ask him if the job is done, it’s not.

“More than anything I’d like to see his passions and his interests and the things he’s been campaigning for come to fruition completely for him.”

Acknowledging that his father’s main destiny in life, as king, is still to come, William - second in line to the throne - adds: “He hasn’t even reached the point that his natural progression should do, i.e. being monarch. So, you know he’s still got his job to do.”

On how he became the man he is today, the Duke of Sussex, who spent ten years in the Army himself, said the Prince could have lived a “very sheltered life” had it not been for his years in the Armed Forces.

Asked whether the Prince’s time in the military, with both the Royal Air Force and the Royal Navy as a young man, shaped him, the Duke of Cambridge said: “No, I’d say it hasn’t.”

“I wouldn’t say it’s shaped him, I’d say it’s helped him form his opinions and his personal discipline and things like that.

“I’d say his shape comes from his passions, from his conservation work and his climate change and he found alternative areas more appealing.”

The Duke of Sussex, who has often spoken of the life-changing benefits his own service gave him, added: “Equally, he could have ended up having a very sheltered life had he not spent that time in the military being with real people and living those fantastic experiences and actually fending for yourself, eating jam sandwiches, living in pretty harsh conditions.